Editing Recurrence relation (section)

===From sequences to grids===
Single-variable or one-dimensional recurrence relations are about sequences (i.e. functions defined on one-dimensional grids). Multi-variable or n-dimensional recurrence relations are about <math>n</math>-dimensional grids. Functions defined on <math>n</math>-grids can also be studied with partial difference equations.<ref>[https://books.google.com/books?id=1klnDGelHGEC Partial difference equations], Sui Sun Cheng, CRC Press, 2003, {{isbn|978-0-415-29884-1}}</ref>